During well testing (drilling) operations and the like, which are carried out from a floating vessel, such as a drillship or semi-submersible, well control is achieved by a sub-sea blowout preventer, which is mounted on the sea-bed to the well head. Such blowout preventers typically comprise a tubular central housing on which are mounted a number of sets of hydraulic rams, for example, four which carry various sealing and cutting tools. The rams are axially spaced along the housing. The lower, or pipe rams, are provided with semi-circular sealing faces, so that when these rams are activated the semi-circular faces mate with the outer surfaces of the well tool. The uppermost set of rams are known as shear rams and are provided with cutting surfaces which can cut through or close the bore of the well tool and isolate the pressurised reservoir fluid from the riser and the upper part of the well tool.
In oil and gas well testing, well pressure control equipment is utilised in addition to the downhole test equipment mounted at the end of the test string, the well pressure control equipment being located above the well head and blowout preventer on the landing string. This equipment provides various safety features and allows for complete well control.
One of the tools utilised in well testing is a sub-sea test tree, a safety valve which is located inside the blowout preventer. During well test operations it is necessary to control both the tubing and annulus pressures, that is the pressure within the string and the pressure between the string and the riser, well casing and well lining. The sub-sea safety tree provides a primary safety system to control tubing pressure and to provide means to disconnect the riser rapidly and safely from the well should adverse conditions occur, such as bad weather or loss of a floating vessels' positioning system. This is partly achieved by providing "fail-safe" valves in the tree, which, for example, are held open during normal operating conditions by supplied hydraulic pressure. If the hydraulic pressure is cut-off, the valves will close, isolating the test string below the tree. An upper portion of the tree may then be unlatched from the lower portion of the tree containing the valves, and the landing string and other well pressure control equipment located above the tree withdrawn.
In situations where a blowout appears likely to occur, the shearing rams of the blowout preventer are activated and seal the string by shearing through the landing string above the sub-sea tree leaving it inside the blowout preventer. The pipe rams are normally extended during well testing and thus also form a seal around the outside of the string. To bring the well back to a safe condition and permit retrieval of the downhole test tools it is necessary to "kill" the live well, such that an uncontrolled flow of fluid will not result when the blowout preventer is opened. This is accomplished by reducing the well pressure, which may be achieved by, for example, pumping a fluid such as barium mud, brine or sea water into the string.